Image display devices are roughly classified into light emitting displays such as a CRT (Cathode Ray Tube) or a PDP (Plasma Display Panel), and non-light emitting displays for displaying an image by changing the light transmittance, such as an LCD (Liquid Crystal Display) incorporated with a liquid crystal panel. Generally, the non-light emitting display is incorporated with a planar illumination device serving as a backlight device on a rear surface thereof, and is operable to display an image by changing the transmitted amount of illumination light to be emitted from the planar illumination device at each pixel. Accordingly, in the non-light emitting display, the planar illumination device greatly affects the electric power consumption, the image quality, the size, and the like of the image display device.
In recent years, an LCD incorporated with LEDs (light emitting diodes) as a backlight device has been developed. Use of LEDs is advantageous in realizing an LCD having high electricity-light conversion efficiency and high color purity, as compared with a conventional cathode ray tube system. Also, it is conceived that use of a laser light source as a backlight device is advantageous in producing an image display device with high electricity-light conversion efficiency and high color purity, as compared with an LED. This is because a laser has high electricity-light conversion efficiency and superior color reproducibility due to a small spectral bandwidth, as compared with an LED.
In an LCD, a liquid crystal panel is operable to change the amount of light transmitted through each pixel by controlling a polarized state of illumination light to be emitted from a planar illumination device. Accordingly, generally, in the LCD, polarized light of illumination light is aligned in one direction while transmitting through a polarization filter or a like device, and light whose polarization is aligned in one direction i.e. linearly polarized light is incident onto the liquid crystal panel.
On the other hand, laser light is generally linearly polarized light. Accordingly, using the laser light as illumination light of LCD enables to eliminate the need of using a polarization filter for LCD, thereby enhancing light use efficiency of LCD. Further, since a laser has a very fast time response, as compared with a cathode ray tube, laser light can be modulated at a high speed. Accordingly, this arrangement is advantageous in eliminating use of a color filter, which has been conventionally used for dividing the colors for the pixels of a liquid crystal panel, and realizing a color field sequential system of turning on light by time-sharing a screen for multiple colors. In the case where the color field sequential system is employed, light loss of a color filter for LCD is eliminated. Accordingly, light use efficiency of LCD can be further enhanced.
Planar illumination devices incorporated with a laser light source are roughly classified into two types i.e. light guiding plate type of obtaining planar illumination light by allowing incidence of laser light through a side portion of a light guiding plate, and direct projection type of directly converting laser light into planar illumination light by using a mirror or a lens. Generally, in the light guiding plate type, laser light is incident through a side portion of a light guiding plate, the incident light is diffused in the interior of the light guiding plate, and the diffused light exits from the light guiding plate into a planar shape. A planar illumination device incorporated with a light guiding plate is proposed in e.g. patent literature 1.
On the other hand, in the direct projection type such as a projector, light is projected into a planar shape. In the direct projection type, since basically, light propagates in the air, a planar illumination device with less light transmission loss and high light use efficiency can be produced, as compared with the light guiding plate type constructed to guide light in the interior of the light guiding plate. An example of an image display device using an optical system of the direct projection type is a rear projection image display device.
In recent years, development of thin image display devices including LCDs has been progressed. In the case where the thickness of LCD is reduced, reducing the thickness of a planar illumination device as a backlight device is essential. There is also a demand for a planar illumination device as a backlight device with high light use efficiency in order to suppress the electric power consumption of LCD.
In a planar illumination device of the direct projection type, since a primary part of an optical path of light is constituted of the air, light loss in the optical path is small, and high light use efficiency is secured. Also, generally, in the direct projection type, there is no need of using an optical element which may cause a variation in polarization, such as a light guiding plate having a light diffusing function. Accordingly, the direct projection type is advantageous in obtaining linearly polarized planar illumination light.
A general optical system of a rear projection image display device for directly projecting an image onto a screen is described as an example of a direct projection image display device. FIG. 26 is a cross-sectional view showing a schematic arrangement of a general rear projection image display device. Light to be emitted from an optical engine 100 is modulated by an image display element 101, and the modulated light is expanded by a projection optical system 102. After the expanded light is reflected on a rear mirror 103, the reflected light is projected onto a screen 104. In the conventional rear projection image display device, since light transmitted through the projection optical system 102 is expanded into a rectangular shape, the overall size of the image display device may be increased.
Patent literature: JP-A 2002-184225